Blending Floor and Background to appear as one
The best way to make your floor blend into your background is to composite the two together from separate layers and feather the edge of the floor layer.

In the node network below I've used a Rectangular Mask node into a Blur node to create the blurred masked that is used to set the alpha channel of the floor layer. 



In this image, you can see the size and orientation of the mask node (indicated by the white outline).



And the final effect:



This technique works best with still images, but can be used with animations if the floor edge does not move a lot in relation to the screen.